In the construction of double or triple pane windows, two pieces of glass are assembled and a perimeter spacer is sealed to each piece of glass, completing a sealed Insulated Glass Unit (IGU). The gas within may be atmospheric air or one of several gases that improve the thermal properties of the IGU. A desiccant material is also inserted along most spacers to absorb moisture that is inadvertently sealed within the IGU.
When an IGU is assembled at a particular altitude and then installed in a building with a different altitude, the net pressure difference from the inside to the outside results in a slight deflection on the glass. If the IGU is installed at a lower altitude than where it was assembled, then the glass panes will tend to deflect inward. If the IGU is installed at a higher elevation than where it was constructed, the glass panes will tend to bulge outward. In either case, a number of problems can result if the deflection is too great. For example, the IGU may experience seal failure, which can allow excessive moisture into the IGU, or a heat-amplifying “magnifying glass” effect when one or more of the panes have a highly reflective (low emissivity) thermal coating applied. These situations can require significant service costs to remedy.
There are some existing methods of addressing the problem of glass pane deflection. One method is to include expanding/retracting bladders, but these bladders must be extremely large to handle the volume of air that must be displaced in larger windows which makes this approach rather impractical. Another method is to include an extremely small capillary tube pierced into the IGU which can be sealed off when the IGU arrives at the installation site. These capillary tubes, however, often allow air of an unknown condition to fill and even cycle within the IGU during shipping resulting in contamination of the IGU. The tubes must also be sealed off in every IGU at the installation site, making the process tedious and more costly.